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1دورية أكاديمية
المؤلفون: Fallah,Mahmoud, Nazeri,Amir. H
المصدر: Journal of Microwaves, Optoelectronics and Electromagnetic Applications v.17 n.4 2018
مصطلحات موضوعية: Electromagnetic wave propagation, Groundwaves, Surface wave propagation, Terrain modeling, Consecutive windows, Finite-difference time-domain (FDTD), Time-domain wave propagation (TDWP)
الوصف: Researchers have always been in face of path loss calculation in different media for applications such as telecommunication link design. Wave propagation calculations in large spaces using the FDTD method is time-consuming and imposes a great computational burden. For this reason, to replace the classical FDTD method for wave propagation simulation and path loss calculation in large spaces, optimized methods, namely TDWP, have been provided. In this paper, the use of the TDWP method for wave propagation simulation and path loss calculation above a terrain is investigated. Longitudinal components of ground-waves are taken into account (including direct waves, ground reflections, and surface waves). Propagation space is longitudinally divided into smaller FDTD windows with finite length. The electromagnetic pulse travels through these windows from left to right to the desired point. But despite its capability in reducing computational burden and increasing processing speed, TDWP has lower precision in instantaneous field simulation and calculation of propagation coefficients, so that results obtained from FDTD and TDWP are clearly different. In this paper, some efficient methods are proposed, which yield an increase in method accuracy.
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2دورية أكاديمية
المؤلفون: Almeida,M.P.C., Mello,L. da Silva, Braz,J.A.C., David,R.P., Castellanos,P.V.G., Lima,F.F.
المصدر: Journal of Microwaves, Optoelectronics and Electromagnetic Applications v.16 n.3 2017
مصطلحات موضوعية: Digital broadcasting, HD Radio, Medium wave propagation, Radio propagation
الوصف: This paper presents the results of measurements of medium wave received field strength, HD Radio audio quality and maximum coverage distances in a dense urban region. Analog AM and digital HD Radio signals broadcasting the same content were simultaneously transmitted in São Paulo, the largest city in Brazil. The received signal intensity and the reception quality were recorded along radial routes of up to 45 km from the transmitter under both mobile and static conditions, enabling the comparative assessment of the digital and analog system performances.
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3دورية أكاديمية
المؤلفون: Ghaffarzadeh,Hosein, Barghian,Majid, Mansouri,Ali, Sadeghi,Morteza. H
المصدر: Latin American Journal of Solids and Structures v.13 n.14 2016
مصطلحات موضوعية: Damage quantification, Wave propagation, Meshfree method, Radial basis function, Hermite Radial point interpolation
الوصف: This paper investigates the numerical modeling of the flexural wave propagation in Euler-Bernoulli beams using the Hermite-type radial point interpolation method (HRPIM) under the damage quantification approach. HRPIM employs radial basis functions (RBFs) and their derivatives for shape function construction as a meshfree technique. The performance of Multiquadric(MQ) RBF to the assessment of the reflection ratio was evaluated. HRPIM signals were compared with the theoretical and finite element responses. Results represent that MQ is a suitable RBF for HRPIM and wave propagation. However, the range of the proper shape parameters is notable. The number of field nodes is the main parameter for accurate wave propagation modeling using HRPIM. The size of support domain should be less thanan upper bound in order to prevent high error. With regard to the number of quadrature points, providing the minimum numbers of points are adequate for the stable solution, but the existence of more points in damage region does not leads to necessarily the accurate responses. It is concluded that the pure HRPIM, without any polynomial terms, is acceptable but considering a few terms will improve the accuracy; even though more terms make the problem unstable and inaccurate.
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4دورية أكاديميةEffects of dust charge variation on electrostatic waves in dusty plasmas with temperature anisotropy
المؤلفون: Juli,M.C. de, Schneide,R.S., Ziebell,L. F., Gaelzer,R.
المصدر: Brazilian Journal of Physics v.39 n.1 2009
مصطلحات موضوعية: Electrostatic waves, Kinetic theory, Magnetized dusty plasmas, Dust charge fluctuation, Wave propagation
الوصف: We utilize a kinetic approach to the problem of wave propagation in dusty plasmas, taking into account the variation of the charge of the dust particles due to inelastic collisions with electrons and ions. The components of the dielectric tensor are written in terms of a finite and an infinite series, containing all effects of harmonics and Larmor radius. The formulation is quite general and valid for the whole range of frequencies above the plasma frequency of the dust particles, which are assumed motionless. The formulation is employed to the study of electrostatic waves propagating along the direction of the ambient magnetic field, in the case for which ions and electrons are described by bi-Maxwellian distributions. The results obtained in a numerical analysis corroborate previous analysis, about the important role played by the dust charge variation, particularly on the imaginary part of the dispersion relation, and about the very minor role played in the case of electrostatic waves by some additional terms appearing in the components of the dielectric tensor, which are entirely due to the occurrence of the dust charge variation.
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5دورية أكاديمية
المؤلفون: Ziebell,L. F., Schneider,R. S., Juli,M. C. de, Gaelzer,R.
المصدر: Brazilian Journal of Physics v.38 n.3a 2008
مصطلحات موضوعية: Langmuir waves, Electrostatic waves, Kinetic theory, Magnetized dusty plasmas, Dust charge fluctuation, Wave propagation
الوصف: A kinetic approach to the problem of wave propagation in dusty plasmas, which takes into account the variation of the charge of the dust particles due to inelastic collisions with electrons and ions, is utilized as a starting point for the development of a new formulation, which writes the components of the dielectric tensor in terms of a finite and an infinite series, containing all effects of harmonics and Larmor radius. The formulation is quite general and valid for the whole range of frequencies above the plasma frequency of the dust particles, which were assumed motionless. The formulation is employed to the study of electrostatic waves propagating along the direction of the ambient magnetic field, in the case for which ions and electrons are described by Maxwellian distributions. The results obtained in a numerical analysis corroborate previous analysis, about the important role played by the inelastic collisions between electrons and ions and the dust particles, particularly on the imaginary part of the dispersion relation. The numerical analysis also show that additional terms in the components of the dielectric tensor, which are entirely due these inelastic collisions, play a very minor role in the case of electrostatic waves, under the conditions considered in the present analysis.
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6دورية أكاديمية
المؤلفون: Izrailev,F. M., Makarov,N. M., Rendón,M.
المصدر: Brazilian Journal of Physics v.36 n.3b 2006
مصطلحات موضوعية: Surface scattering, Wave propagation in random guiding media, Electronic transport in quantum wires
الوصف: In this work we study the surface scattering mechanisms from rough surfaces of a multimode quasi-1D waveguide (conducting quantum wires). The square-gradient scattering mechanism, which was missed in existing studies of the transport through surface-corrugated waveguides, is discovered. The main attention is paid to the interplay between the new mechanism and the known one, as well as its effect on the waveguide transport properties. For any value of the roughness height sigma, the square-gradient terms in the expression for the wave-scattering length (electron mean-free path) are dominant, provided the correlation length Rc of the surface disorder is small enough.
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7دورية أكاديمية
المؤلفون: Schneider,R. S., Ziebell,L. F., Juli,M. C. de, Jatenco-Pereira,V.
المصدر: Brazilian Journal of Physics v.36 n.3a 2006
مصطلحات موضوعية: Langmuir waves, Electrostatic waves, Kinetic theory, Magnetized dusty plasmas, Dust charge fluctuation, Wave propagation
الوصف: A kinetic formulation developed to analyze wave propagation in dusty plasmas, which takes into account the variation of the charge of the dust particles due to inelastic collisions with electrons and ions, is utilized to study the propagation and damping of electrostatic waves with wave number exactly parallel to the external magnetic field and Maxwellian distributions for the electrons and ions in the equilibrium. It is shown that, due to the presence of the dust, the damping of Langmuir waves in the region of large wavelengths is increased as compared to conventional Landau damping. Langmuir waves in the occurrence of collisional charging of dust particles also feature weak damping for small wavelengths, which vanishes if the effect of collisional charging of the dust particles is neglected in the dispersion relation. It is also shown that the damping of ion-acoustic waves is modified by the presence of the dust, and that some damping effect due to the dust particles remains even if the effect of collisional charging of dust particles is neglected in the dispersion relation.
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8دورية أكاديمية
المؤلفون: Souza,A.L., Cruz,S.L., Pereira,J.F.R.
المصدر: Brazilian Journal of Chemical Engineering v.17 n.4-7 2000
مصطلحات موضوعية: leak detection, pressure wave propagation, spectral analysis
الوصف: The development and test of a technique for leak detection in pipelines is presented. The technique is based on the spectral analysis of pressure signals measured in pipeline sections where the formation of stationary waves is favoured, allowing leakage detection during the start/stop of pumps. Experimental tests were performed in a 1250 m long pipeline for various operational conditions of the pipeline (liquid flow rate and leakage configuration). Pressure transients were obtained by four transducers connected to a PC computer. The obtained results show that the spectral analysis of pressure transients, together with the knowledge of reflection points provide a simple and efficient way of identifying leaks during the start/stop of pumps in pipelines.
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9دورية أكاديمية
المؤلفون: Helbig,K.
المصدر: Revista Brasileira de Geofísica v.18 n.2 2000
مصطلحات موضوعية: Elastic anisotropy, Transverse anisotropy, Exploration seismics, Wave propagation
الوصف: In Part 1 of this paper (Helbig, 1998 - Rev. Bras. Geof. 16 (2 - 3):103 - 114) it was shown that a medium consisting of a periodic sequence of layers is, in the long-wavelength approximation, equivalent to a homogeneous compound medium with elastic parameters that are generalized averages of the constituents' stiffnesses. Though the matrix-algorithm described in Part 1 works with anisotropic constituents, the most interesting application is to layer sequences with isotropic constituents, i.e., to transversely isotropic (TI) compound media. Part 2 discusses the possibility to obtain information about the (thin-layer) constituents from the properties of the compound medium. Though every periodic sequence of isotropic layers results in a TI medium, the reverse is not true: there are TI media that cannot be "modeled" by a periodic sequence of isotropic layers. Those that can be modeled can be inverted to layer sequences that result in precisely the observed anisotropy. This inversion is not unique, but it constrains the possibilities. The critical tool to determine the possibility of modeling a TI medium is the concept of stability. Unstable compound media - that release energy on being deformed - would not exist. However, for inversion we must insist that not only the compound medium, but also the potential constituents are stable. In preparing a catalog that covers all possible media, instability is the boundary beyond which the calculation becomes meaningless. Inversion means to determine possible causes of the observed anisotropy, ideally the elastic parameters of the constituents and their contribution to the compound medium. This is possible, though under several restrictions: Not all TI media are long-wave equivalent to a periodically layered sequence of isotropic layers. Those that are can be "modeled" by a variety of layer sequences. Every TI medium that can be modeled at all can be modeled by as few as three layers, but the set of all models is a three-parametric manifold. If a TI medium can be modeled by ...
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10دورية أكاديمية
المؤلفون: Helbig,Klaus
المصدر: Revista Brasileira de Geofísica v.16 n.2-3 1998
مصطلحات موضوعية: Elastic anisotropy, Exploration seismics, Wave propagation
الوصف: An important reason for elastic anisotropy of geological media is layering on a scale that is small to the wavelengths. If the parameters of the layering are known, the "effective parameters" of the compound medium can be derived from the parameters of the constituents. If the layering is periodic with the period small compared to the wavelengths, the compound medium can be replaced by a homogeneous medium. One way to achieve this description is to re-formulate Hooke's law. First, the six components of stress and strain that vary from constituent to constituent are expressed in terms of the six components that are, in view of the continuity condition, constant throughout the layer. Second, the effective stress- and strain components are determined as the thickness-weighted arithmetic averages of the corresponding constituents' components. Third, the standard form of Hooke's law (stress as a function of strain) is re-established. This algorithm can be applied to constituents of any type. For seismic applications, the most important layering is that of isotropic layers. There are two significant situations that afford a simplification: (i) if all constituents have the same shear stiffness, the replacement medium is homogeneous; (ii) if all constituents have the same Poisson's ratio, wave propagation in a relatively wide cone about the axis of symmetry cannot be distinguished from wave propagation in an isotropic medium.
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